Nirmal Punjabi

Plasmonic-ELISA: expanding horizons

The enzyme-linked immunosorbent assay (ELISA) has been an integral part of in vitro diagnostic tests due to its high specificity, standard configuration, and convenient readout. The convergence of the plasmonic property, i.e. localized surface plasmon resonance (LSPR), of noble metal nanoparticles with the ELISA technique has established a novel category of immunoassay, known as “plasmonic ELISA” (pELISA). It has enabled the naked eye detection of various disease biomarkers down to attomolar concentration. The color development relies on the biocatalytic reaction mediated modulation of LSPR properties. Through this review we present various current state-of-art pELISA strategies adopted for naked-eye quantification of disease biomarkers along with their advantages, limitations and applicability. These strategies are broadly classified into following four categories based on the different means of LSPR modulation: (i) aggregation, (ii) controlled growth kinetics, (iii) metallization, and (iv) etching of metal nanoparticles. Furthermore, the paper highlights the different biocatalytic reactions and their role in plasmonic color development as a function of analyte concentration. The review ends with an elucidation of current challenges and future perspectives of pELISA precisely focusing on the need for development of next generation low cost point-of-care diagnostic kits.

Evanescent Wave Absorption Based Fiber-Optic Sensor - Cascading of Bend and Tapered Geometry for Enhanced Sensitivity

Evanescent wave absorption (EWA) based fiber-optic sensors have found widespread applications ranging from environmental sensing to biosensing. In these sensors, optical and geometrical characteristics such as optical fiber type (single-mode or multi-mode), fiber core diameter, fiber probe geometry, fiber probe length, etc., are very important. These parameters affect the penetration depth and fractional power by modulating the ray propagating in the fiber probe that ultimately influences the sensitivity of the EWA sensors. Various geometries of fiber probe designs, like bent, tapered, coiled, etc., have been explored for improving the sensitivity. This chapter describes the design, development and fabrication of a novel bent-tapered fiber-optic sensor. A combination of bending and tapering acts as a mode converter, which results in high penetration depth of the evanescent field. In addition, tapered region of the probe increases the coupling efficiency at the detector end by V-number matching and thus improves the signal-to-noise ratio. EWA sensitivity of the sensor was compared for different taper ratios. Finally, the optimized geometrical design was used to demonstrate biosensing application.

Micromolded U-shaped PDMS optical waveguide for biosensing applications

Integrated optical waveguide sensors are usually fabricated using materials like silicon, silica, SU-8, etc. Their fabrication requires clean room processes which are expensive and time-consuming. We demonstrated the fabrication of PDMS based optical waveguide in non-cleanroom environment using soft lithography technique. A master-mold was fabricated using Acralyn. PDMS polymer was chosen for waveguide fabrication, as it provides low refractive index contrast in the sensing region. These PDMS waveguides were found to be 5-times more sensitive than SU-8 waveguides. High sensitivity along with mechanical robustness and ease of fabrication of PDMS waveguides provides a promising and versatile platform for biosensor application.

Design and Fabrication of Lossy Mode Resonance Based U-Shaped Fiber Optic Refractometer Utilizing Dual Sensing Phenomenon

The present work for the first time, provides a detailed analysis of U-shaped lossy mode resonance (U-LMR) fiber optic refractometer which is based on dual sensing phenomena. Theoretical and experimental studies on the sensitivities of U-LMR based fiber optic refractometer have been carried out utilizing different surrounding medium refractive indices (SRI). Also, the effect of bend diameter on the sensitivity has been investigated in detail by fabricating and analyzing zinc oxide (ZnO) coated U-shaped fiber optic probes of various bend diameters. Experimental sensitivities of the proposed U-LMR fiber optic refractometer were characterized in terms of wavelength shift, absorbance and fullwidth at half-maximum (FWHM) changes with respect to the SRI. With ZnO coated U-shaped fiber optic probe of bend diameter of 1.5 mm, we achieved six fold increase in the LMR sensitivity compared to the straight LMR probe. Also, in terms of the absorbance, our proposed probe was found to be four times sensitive compared to the uncoated U-shaped probe. In addition to this, for the probe diameter of 2.1 mm, sensitivity obtained by monitoring a new parameter, viz. FWHM change, is ~ 900 nm/RIU which is at least 3 times better than the straight LMR probe.

A novel 'Gold on Gold' biosensing scheme for an on-fiber immunoassay

In this paper, we propose a novel „gold on gold‟ biosensing scheme for absorbance based fiber-optic biosensor. First, a self-assembled monolayer of gold nanoparticles is formed at the sensing region of the fiber-optic probe by incubating an amino-silanized probe in a colloidal gold solution. Thereafter, the receptor moieties, i.e. Human immunoglobulin G (HIgG) were immobilized by using standard alkanethiol and classic carbodiimide coupling chemistry. Finally, biosensing experiments were performed with different concentrations of gold nanoparticle-tagged analyte, i.e. Goat anti- Human immunoglobulin G (Nanogold-GaHIgG). The sensor response was observed to be more than five-fold compared to the control bioassay, in which the sensor matrix was devoid of gold nanoparticle film. Also, the response was found to be ~10 times higher compared to the FITC-tagged scheme and ~14.5 times better compared to untagged scheme. This novel scheme also demonstrated the potential in improving the limit of detection for the fiber-optic biosensors.

Dendrimer as a multifunctional capping agent for metal nanoparticles for use in bioimaging, drug delivery and sensor applications

Advances in nanoparticle research, particularly in the domain of surface-engineered, function-oriented nanoparticles, have had a profound effect in many areas of scientific research and aided in bringing unprecedented developments forward, particularly in the biomedical field. Surface modifiers/capping agents have a direct bearing on the major properties of metal nanoparticles (MNPs), ranging from their physico-chemical properties to their stability and functional applications. Among the different classes of capping agents, dendrimers have gained traction as effective multifunctional capping agents for MNPs due to their unique structural qualities, dendritic effect and polydentate nature. Dendrimer-coated metal nanoparticles (DC-MNPs) are typically produced by both (i) a one-pot strategy, where metal ions are reduced in the presence of dendrimer molecules and (ii) a multi-pot strategy, where a sequence of reactions involving the reduction of metal ions, activation, conjugation and purification steps are involved. These DC-MNPs have shown remarkable ability to stabilize MNPs by means of electrostatic interactions, coordination chemistry or covalent attachment, due to them entailing a large number of sites at which further molecular moieties can be conjugated. This review article is an attempt to consolidate the on-going work, particularly over the last five years, in the field of the synthesis of dendrimer-coated MNPs and their potential applications in bioimaging, drug delivery and biochemical sensors.

Detection of bacteria using bacteriophage with hollow gold nanostructures immobilized fiber optic sensor

Hollow gold nanostructures (HGNS) have been used in variety of optical biosensors due to their inherent advantage of operating at near infra red (NIR) wavelength, large extinction coefficient and high dielectric sensitivity. The absorption wavelength of these nanostructures can be modulated by changing the ratio of hollow region to the core shell thickness. The aim of the present study is to incorporate the properties of HGNS, to develop LSPR based U-bent fiber optic sensor for detection of pathogens. The detection was carried out using an experimental set up consisting of a white light source, 200 μm diameter optical fiber having bend diameter of 1.6 mm ± 0. 2 mm and a spectrometer. The HGNS were immobilized on the decladded portion of the fiber optic probe by chemisorptions. The effective plasmon penetration depth of the HGNS immobilized fiber optic sensor was approximated by using alternating layers of positively and negatively charged polyelectrolytes. The HGNS immobilized U-bent fiber optic sensor was used for detection of E.coli B40 strain using bacteriophage T4. The preliminary experiments were carried out with 104 cfu/ml of E.coli B40 and the change in absorbance obtained was approx. 0.042 ± 0.0045 abs. units (n = 3). The response of this sensor was found to be better than spherical gold nanoparticle immobilized sensing platforms.

Fabrication of Out-of-Plane Multi-Bend Fiber-Optic Sensor for Enhanced Refractive Index Sensing

We designed a simple fabrication scheme to produce out of plane multi-bend fiber-optic sensor. This geometry showed significant enhancement in sensitivity, along with sturdiness and ease of usage compared to other reported fiber geometry.

Detection of α-Synuclein, Marker for Parkinson’s disease using Localized Surface Plasmon Resonance Fiber Optic Sensor

Localized surface plasmon resonance (LSPR) based fiber optic sensor probe is fabricated and is used to detect α-Synuclein, a marker for Parkinson’s disease. The preliminary studies show the Limit of detection of 70nM.

FITC doped fluorescent silica nanoparticles based optical fiber sensor for copper (II) detection

We have demonstrated a fast, sensitive fluorescent tapered tip optical fiber (TTOF) sensor to detect copper (Cu (II)) ions. The principle of detection is based on fluorescent quenching of fluorescein isothiocynate (FITC) doped fluorescent silica nanoparticles (FSi) by Cu(II) ions. TTOF was functionalized with FITC doped fluorescent silica nanoparticles (FSi), as transduction element. Amine functionalized FSi nanoparticles are immobilized on aminosilanised tapered tip using glutaraldehyde treatment. Cu (II) ions which form chelation with amines quench the fluorescence (FL) of the FSi nanoparticles. Detection capacity down to 0.25 μM for copper ions has been achieved in this configuration.